Well, in my country. You can be 12-13 in year 7.
 
.-. But it in this case I don't really know. Because most of the characters are like 16+ (Dangan Series)
(Except Saionji but i have no idea how freaking old she is like 9?)Was referring to the chick in Super Dangan Ronpa 2)

Spose it would depend on your characters Ultimate talent tbh

Edit: I had an IDEAAAA. 
Perhaps your character can be from another class, but we find you kept alive with us in the school? Yes no maybe? Then the age difference wouldn't particularly matter~